Method and apparatus for affine based inter prediction of chroma subblocks

A decoding apparatus, comprising:

• a video data memory;
• and
• a video decoder configured to:
• perform an image decoding process on an encoded bitstream, to obtain a decoded image, wherein the decoded image comprises an image block coded in an inter prediction mode, wherein the image block comprises a luma block and a co-located chroma block, and wherein the performing the image decoding process comprises a chroma motion vector derivation process, comprising:
• obtaining an averaged luma motion vector mvAvgLX by:
• determining chroma scaling factors in horizontal and vertical directions based on chroma format information, wherein the chroma format information indicates a chroma format of a current picture which the image block belongs to, wherein the chroma format comprises one of a YUV 4:2:2 format, a YUV 4:2:0 format or a YUV 4:4:4 format;
• determining a set(S) of luma subblocks of the luma block based on values of the chroma scaling factors;
• and
• generating the averaged luma motion vector mvAvgLX by averaging motion vectors of the luma subblocks in the set(S), in a case that more than one luma subblocks are in the set(S);
• and
• deriving a chroma motion vector for a chroma subblock mvCLX[xSbIdx][ySbIdx] of the chroma block based on the averaged luma motion vector mvAvgLX;
• wherein a process of deriving the chroma motion vector is invoked as follows with the mvAvgLX as input, and the chroma motion vector mvCLX[xSbIdx][ySbIdx] as output:
• mvCLX
• [
• 0
• ]
• =
• mvLX
• [
• 0
• ]
• *
• 2
• /
• SubWidthC
• mvCLX[1]=mvLX[1]*2/SubHeightC, if a reference picture corresponding to a reference index value refIdxLX for the image block is not a current picture;
• or
• mvCLX
• [
• 0
• ]
• =
• (
• (
• mvLX
• [
• 0
• ]
• ≫
• (
• 3
• +
• SubWidthC
• )
• )
• *
• 32
• mvCLX[1]=((mvLX[1]>>(3+SubHeightC))*32, if the reference picture corresponding to the reference index value refIdxLX for the image block is the current picture,
• wherein mvLX[0] corresponds to a horizontal component of the mvAvgLX, and mvLX[1] corresponds to a vertical component of the mvAvgLX,
• wherein when both SubWidthC and SubHeightC are equal to 1, the set of luma subblocks(S) is a luma subblock indexed by S0=(xSbIdx, ySbIdx);
• when at least one of SubWidthC and SubHeightC is not equal to 1, the set of luma subblocks(S) is:
• a first luma subblock indexed by S0=
• (
• (
• xSbIdx
• ≫
• (
• SubWidthC
• -
• 1
• )
• ≪
• (
• SubWidthC
• -
• 1
• )
• )
• ,
• (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))), and a second luma subblock indexed by
• S
• 1
• =
• (
• (
• xSbIdx
• ≫
• (
• SubWidthC
• -
• 1
• )
• ≪
• (
• SubWidthC
• -
• 1
• )
• )
• +
• (
• SubWidthC
• -
• 1
• )
• ,
• (
• ySbIdx
• ≫
• (
• SubHeightC
• -
• 1
• )
• ≪
• (
• SubHeightC
• -
• 1
• )
• )
• +
• (
• SubHeightC
• -
• 1
• )
• )
• wherein SubWidthC and SubHeightC represent the chroma scaling factors in horizontal and vertical directions, respectively;
• xSbIdx and ySbIdx represent a subblock index in the horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set(S);
• “<<” represents a left arithmetic shift;
• “>>” represents a right arithmetic shift;
• xSbIdx is an integer number ranging from 0 to numSbX−1 and ySbIdx is an integer ranging from 0 to numSbY−1;
• numSbX indicates the number of luma subblocks in the luma block along the horizontal direction;
• and
• numSbY indicates the number of luma subblocks in the luma block along the vertical direction.

A non-transitory computer-readable medium storing program instructions that, when executed by one or more processors, cause the one or more processors to perform operations of chroma motion vector derivation used in an affine based inter prediction of a current image block comprises a luma block and a co-located chroma block, the operations comprising:

• performing an image decoding process on an encoded bitstream, to obtain a decoded image, wherein the decoded image comprises an image block coded in an inter prediction mode, wherein the image block comprises a luma block and a co-located chroma block, and wherein the performing the image decoding process comprises a chroma motion vector derivation process, comprising:
• obtaining an averaged luma motion vector mvAvgLX by:
• determining chroma scaling factors in horizontal and vertical directions based on chroma format information, wherein the chroma format information indicates a chroma format of a current picture which the image block belongs to, wherein the chroma format comprises one of a YUV 4:2:2 format, a YUV 4:2:0 format or a YUV 4:4:4 format;
• determining a set(S) of luma subblocks of the luma block based on values of the chroma scaling factors;
• and
• generating the averaged luma motion vector mvAvgLX by averaging motion vectors of the luma subblocks in the set(S), in a case that more than one luma subblocks are in the set(S);
• deriving a chroma motion vector for a chroma subblock mvCLX[xSbIdx][ySbIdx] of the chroma block based on the averaged luma motion vector mvAvgLX;
• wherein a process of deriving the chroma motion vector is invoked as follows with the mvAvgLX as input, and the chroma motion vector mvCLX[xSbIdx][ySbIdx] as output:
• mvCLX
• [
• 0
• ]
• =
• mvLX
• [
• 0
• ]
• *
• 2
• /
• SubWidthC
• mvCLX[1]=mvLX[1]*2/SubHeightC, if a reference picture corresponding to a reference index value-refIdxLX for the image block is not a current picture;
• or
• mvCLX
• [
• 0
• ]
• =
• (
• (
• mvLX
• [
• 0
• ]
• ≫
• (
• 3
• +
• SubWidthC
• )
• )
• *
• 32
• mvCLX[1]=((mvLX[1]>>(3+SubHeightC))*32, if the reference picture corresponding to the reference index value refIdxLX for the image block is the current picture,
• wherein mvLX[0] corresponds to a horizontal component of the mvAvgLX, and mvLX[1] corresponds to a vertical component of the mvAvgLX,
• wherein when both SubWidthC and SubHeightC are equal to 1, the set of luma subblocks(S) is a luma subblock indexed by S0=(xSbIdx, ySbIdx);
• when at least one of SubWidthC and SubHeightC is not equal to 1, the set of luma subblocks(S) is:
• a first luma subblock indexed by S0=
• (
• xSbIdx
• ≫
• (
• SubWidthC
• -
• 1
• )
• ≪
• (
• SubWidthC
• -
• 1
• )
• )
• ,
• (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))), and a second luma subblock indexed by
• S
• 1
• =
• (
• (
• xSbIdx
• ≫
• (
• SubWidthC
• -
• 1
• )
• ≪
• (
• SubWidthC
• -
• 1
• )
• )
• +
• (
• SubWidthC
• -
• 1
• )
• ,
• (
• ySbIdx
• ≫
• (
• SubHeightC
• -
• 1
• )
• ≪
• (
• SubHeightC
• -
• 1
• )
• )
• +
• (
• SubHeightC
• -
• 1
• )
• )
• wherein SubWidthC and SubHeightC represent the chroma scaling factors in horizontal and vertical directions, respectively;
• xSbIdx and ySbIdx represent a subblock index in the horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set(S);
• “<<” represents a left arithmetic shift;
• “>>” represents a right arithmetic shift;
• xSbIdx is an integer number ranging from 0 to numSbX−1 and ySbIdx is an integer ranging from 0 to numSbY−1;
• numSbX indicates the number of luma subblocks in the luma block along the horizontal direction;
• and
• numSbY indicates the number of luma subblocks in the luma block along the vertical direction.